This invention relates to the roughening or graining and anodizing of smooth planar metal surfaces and especially to the preparation of a grained anodized aluminum base for lithographic printing plates.
The art of lithographic printing depends upon the immiscibility of greasy substances (ink) and water and upon the preferential retention of the greasy ink by an image area on the plate and of an aqueous dampening fluid by the non-image area. Upon application of ink to the plate, the image portion retains the ink whereas the moistened non-image area repels the ink. The lithographic printing plate has a coating of a light-sensitive substance that is adherent to the anodized aluminum base sheet.
In coating an aluminum base plate with a light-sensitive material, it is desirable to provide an adherent, hydrophilic, abrasion-resistant surface. This is best achieved by graining or roughening the surface to provide a large surface area, anodizing to provide a hard surface resistant to abrasion and corrosion, and subsequently treating with a material selected to enhance the adhesion of the coating to the base layer, improve hydrophilicity and/or improve developability of the plate. Graining can be carried out by mechanically treating the aluminum, for example by brush graining or ball graining, or it can be grained chemically or electrochemically. Although electrochemical graining produces a very desirable grained surface, slurry brush graining produces a satisfactory grained surface at a significantly lower cost.
Slurry brush graining is normally carried out continuously on a moving aluminum web using a plurality of rotating brushes with an aqueous slurry of abrasive particles fed from recirculating sumps. The abrasive particles may, for example, be quartz, silica, pumice or unfused crystalline alumina.
The post anodic treatment can be done by various methods. U.S. Pat. No. 3,181,461 discloses the use of an aqueous sodium silicate. A treatment with polyvinyl phosphonic acid is disclosed in U.S. Pat. No. 3,276,868. The present invention may be utilized with these or any of the post treatment methods normally employed in conjunction with a typical anodizing process.